1. Field of the Invention
The invention relates to the treatment of sludge with a high content of fermentable organic matter, in particular sludge derived from processes for urban and industrial waste water treatment, drinking water treatments using surface or ground water, or sludge derived from network cleaning operations. The invention also applies to the treatment of other waste from treatment facilities such as screen debris, fats and sand.
More precisely, the invention relates to a process for stabilizing such sludge or waste, that is to say in particular for its deodorization or sanitization.
In France, the quantity of sludge produced by wastewater and drinking water plants is over one million tonnes of dry matter per year. Approximately, one half of this sludge is put to agricultural use while 35% is stored in waste tips. As the production of this sludge is on the increase, it should not cause a hazard for the environment or human health. This sludge contains germs of which some are pathogenic (Coliform bacteria, salmonellae, helminth eggs. . . ). In addition, it is highly fermentable and is a source of gas generation (amines, hydrogen sulphide, mercaptans) giving rise to olfactory nuisances.
These considerations account for the necessity, for the above-mentioned treatment sectors, to make provision for at least one stage of sludge stabilization intended to obtain sludge that is no longer undergoing change, or at least undergoing very slow change, both at biological level and at physico-chemical level.
This sludge is made up of a soluble part and a dry matter part. The dry matter comprises approximately 30% mineral compounds and approximately 70% of so-called "volatile" organic matter. Traditionally, the main objective of sludge stabilization treatments is in particular to reduce its fermentable ability by oxidizing the organic matter for its improved bio-assimilation or by eliminating the reducing germs which are the cause of fermentation.
As specified above, sludge stabilization treatments are also intended to meet two main objectives:
sludge sanitization by reducing the number of potentially pathogenic germs; PA1 sludge deodorization by reducing its fermentable capacity, that is to say by reducing the number of reducing bacteria (in particular sulphato-reducing germs) and/or by reducing organic matter concentration. PA1 aerobic digestion PA1 anaerobic digestion PA1 chemical packaging PA1 thermal packaging PA1 one is a stabilization process known to men of the art under the name SYNOX PA1 the other is a stabilization process using gaseous nitrogen oxides. PA1 pretreatment of sludge including a decrease in pH of between 2.5 and 3, adjusting dry matter content up to 2.5-3.5% by weight, and heating the sludge to a temperature of 40.degree. C.; PA1 oxidation of the sludge by contact with 5% ozonated air for 30 to 60 minutes, with a view to reducing its organic matter concentration; pre-treatment and oxidation are conducted in a reactor under pressure (6 to 9 bars); PA1 optional post-treatment using sodium nitrite which is added to reduce the concentration of viruses. PA1 long stay lengths (aerobic and anaerobic digestion) PA1 production of malodorous gases (anaerobic digestion leading to biogas release, quickliming which produces ammonia, thermal packaging requiring high energy consumption and generating volatile fatty acids). PA1 increase in sludge volume--quickliming) PA1 limited time duration of stabilization PA1 high complexity and high implementation costs PA1 sometimes mediocre performance regarding deodorization and sanitization. PA1 complete deodorization of the sludge (inhibited formation of H.sub.2 S, CH.sub.3 SH and organic sulphides), PA1 elimination of faecal contamination germs (total and thermotolerant Coliforms, streptococci), PA1 elimination of parasitic eggs (helminth eggs). PA1 complete deodorization of the sludge (inhibited formation of H.sub.2 S, CH.sub.3 SH and organic sulphides), PA1 elimination of faecal contamination germs (total and thermotolerant Coliforms, stroptococci), PA1 elimination of parasitic eggs (helminth eggs, Ascaris, Taenia, Trichuris. . ), and also, PA1 elimination of spore-forming bacteria (Clostridium spores . . ), PA1 elimination of Protozoa (Giardia, Cryptosporidium . . . ).
Also, the stabilization stage must, for reasons relating to operating costs, cause the least possible deterioration to sludge dehydrating capacity This parameter has a direct effect on polymer consumption required by treatment processes to convert sludge to other uses or to store as waste.
2. Description of the Prior Art
There are numerous stabilization processes in the state of the art for such sludge. They are essentially classified as follows:
More recently, two new processes were examined but have not yet, as at the date of filing of this present application, been properly developed at industrial level.
All these processes can be applied to sludge having a relatively high concentration of dry matter, frequently between 50 and 100 g/l. This therefore requires that this sludge is submitted to preliminary thickening treatment generally by settling or flotation. Treatment facilities must therefore be designed so that they are of sufficient size to allow a minimum contact time of 24 hours.
Aerobic digestion reduces organic matter concentration and consists of extended aeration of sludge so that the development of the aerobic microorganisms it contains can be continued until elimination of the organic matter substrate, achieving self-oxidation of these microorganisms.
Aerobic stabilization can be conducted at 30.degree.-40.degree. C. (mesophil stabilization) or at 45.degree.-60.degree. C. (thermophil stabilization). In the latter case, the temperature increase leads to partial sanitization of the sludge.
The stay length of the sludge in aerobic stabilization reactors is generally 10 to 15 days which achieves 30 to 40% elimination of organic matter, 90-96% elimination of sulphurated matter emission, and a reduction of over 2 log in faecal germs. If the facilities in which this aerobic stabilization is carried out are covered and insulated, the degradation of organic matter produces an increase in temperature which accelerates the digestion process. The length of stay can therefore be reduced by half, down to 5 to 6 days.
Anaerobic stabilization consists of degrading organic matter with strictly anaerobic bacteria under specific environmental conditions ( redox potential in the region of--250 mV, neutral pH) until the formation of methane. Methanisation of the organic matter can take place under ambient, mesophil or thermophil conditions and requires a stay length of 20 to 30 days. Under these conditions, degradation of organic matter generally reaches 40 to 50%. The odour index is reduced by 70 to 80% and pathogenic germs by 1 to 2 log.
Chemical packaging using lime is used at different stages of sludge treatment. Lime (quicklime) can trap the water contained in the sludge during a disintegration reaction and greatly increases its siccative properties. In addition to this increased drying, quicklime plays a powerful stabilizing role through strong alkalinization of the medium, which allows extensive destruction of pathogenic agents (sanitization) and of the microorganisms responsible for anaerobic fermentation leading to a decrease in the emission of sulphur pollutants. The use of quicklime generally enables faecal germs to be reduced by 4 to 5 log and the flow of sulphurated molecules (H.sub.2 S, mercaptans) by over 90%. However, the rise in pH which results from its use causes the release of ammonia and therefore an olfactory nuisance. Also, the addition of quicklime leads to an increase in sludge mass in proportion to treatment level of at least 30%. Moreover, the stabilization obtained with quicklime is fairly short-lived, generally 8 to 30 days according to the proportion of quicklime added.
Sludge treatment by thermal packaging consists of treating sludge at high temperature (150.degree.-220.degree. C.) under high pressure (15 to 20 bars) for 30 to 40 minutes to achieve sanitization and partial conversion of the organic matter it contains into biodegradable, soluble compounds (alcohols, aldehydes, volatile fatty acids) so as to destroy the colloid slats and thereby improve subsequent stages of thickening and dehydration. The main drawback of the process lies in the olfactory nuisances generated by resulting organic molecules such as butyraldehydes, butyric compounds, valeraldehydes. . . and its very high energy cost which means that this process is increasingly less used.
Among the above-mentioned processes recently developed, the SYNOX process consists of treating raw sludge in several stages comprising:
The main disadvantage of this process is that it is very complex both in design and implementation. It requires at least two reactors and is most costly from an energy viewpoint since it includes the stages of O.sub.3 production, heating, and pressurizing.
Another process described in French Patent n.degree. 9310335 filed under the name of the Applicant, recommends the use of oxides of gaseous nitrogen for sludge stabilization.
This process consists of transferring nitrogen oxides to a liquid sludge (nitrogen monoxide, nitrogen dioxide, nitrogen protoxide. . . ) that are either pure or diluted in a mixture with another gas, and of assuring a contact time of less than 5 hours between the sludge and the nitrogen oxides.
Although it achieves very good levels of performance in respect of sludge deodorization (odour reduction: 3 to 4 log) and sanitization (germ reduction: 4 log) within a much reduced contact time (a few hours) compared with sludge stabilization processes (20 to 30 days for anaerobic stabilization, approximately 10 days for thermophil aerobic stabilization), this latter process using nitrogen oxides nevertheless has two disadvantages.
Firstly, this process implies the presence, in the air exiting the reactor, of residual nitrogen oxide which must be eliminated, necessitating fairly costly additional treatments.
Also, this process implies relatively high investment costs connected with the production of NO.sub.x, in particular in the case of in situ production, by thermal oxidation of ammonia.
In conclusion, the disadvantages of known sludge stabilization processes already in operation or still only at the development stage are as follows:
Also, these processes frequently lead to refermenting of the stabilized sludge over time and during storage in areas provided for this purpose. Table 1 below summarises the performances of the chief processes in the current state of technique:
TABLE 1 __________________________________________________________________________ Anaerobic Aerobic digestion stabilization Quickliming SYNOX NOX __________________________________________________________________________ Type of thickened thickened dehydrated raw raw sludge Length of stay 20 to 30 6-12 days -- 60' to several 4 h days hours Volatile m. 45-50% 35-40% -- -- -- variation Deodorization H2S:1-2 log H2S:2-3/4-5 log. H2S &gt;5 log -- H2S = 5 log CH3SH = 4 log Sanitization T &lt; 30.degree./T &gt; 50.degree. C. Coliform 1.5 log 0.5 to 1/3 to 5 3 log 5 log &gt;5-6 log Streptococci 1.5 log log 3 log &gt;5-6 log 3 log Clostrid. Sp. &lt;0.5 log 0.5 to 1/3 to 5 -- &gt;4-5 log 1 log log 3 to 5 log __________________________________________________________________________